1. Field of the Invention
This invention relates to an apparatus for cutting molten strands of extruded thermoplastic into pellets.
More specifically, the invention is an improved rotary cutter apparatus whose knife-blades cut strands of molten polymer extruding from a die, each knife-blade cutting-edge being independently maintained in contact with the die-face by means of a controlled spring mechanism.
2. Description of Related Art
Polymer pellets or granules are commonly prepared by cutting strands of polymer emerging from an extruder die. This may be done by first allowing the strand of molten polymer to solidify or by cutting the strand while it is molten. The latter has been referred to as a die-face cutter or melt cutter or melt cutter apparatus. Solid polymer strands are cut some distance away from the die-face, after they have solidified, but molten polymer strands are conveniently cut by a knife-blade whose cutting-edge is in contact with the die-face while moving across it.
Melt cutters are commonly of rotary design. Generally, but not necessarily, the extruder die-face and the cutter are housed within cooling water, so that cut molten pellets quickly solidify and don't stick together. High capacity can be achieved by such underwater operation.
One key to good cutting action in such cutters is the maintenance of low, essentially zero clearance between the cutting-edges and the die-face. If the knife-blade position is rigidly fixed, die contraction due to changes in water and polymer melt temperature, die warping, and other factors could allow the clearance to become significantly greater than zero or, by contrast, die expansion could cause excessive die-face and knife-blade cutting-edge wear. When the clearance becomes significantly greater than zero, cutting action is inferior or even totally interrupted. Lack of smooth continuous cutter operation greatly affects productivity. Many different designs and mechanisms, usually quite complex, have been used to achieve this knife-blade cutting edge/die-face adjacency.
U.S. Pat. No. 4,179,255 (Hale), discloses a rotary melt cutter operating in association with a rotating shaft structure. The cutter has several knife-assembles each of which have two movable joints. Hydraulic pressure on knife-blades of the knife-assemblies urges the knife-blades towards the die concomitant with rotation of one of those joints. Several features of the Hale design are also part of the improved cutter of this invention and the patent is herein incorporated by reference.
U.S. Pat. No. 4,978,288 (Elwood) discloses a rotary cutter with an improved pellet transport system. The cutter has an annular chamber in which blade means comprising a knife-holder having articulated self-adjusting spring-loaded knives are disposed. The blades can pivot about two mutually transverse axes and are urged against the pellet plate by the spring.
U.S. Pat. No. 4,019,414 (Thomas, Jr. et al.) discloses a rotary melt cutter operating under water, having each of several cutting knives attached in cooperation with a spring adjacent to each knife, to a rotating structure to form a cutting assembly. Wear on the blade is reduced by means of a cam action which moves each blade radially, to expose a continually differing part of each blade to a cutting position.
U.S. Pat. No. 3,196,487 (Snelling) discloses a rotating cutting assembly which employs a hub structure. The rotating knife portion of the assembly is urged towards the die-face preferably by a single spring positioned centrally along the axis of the drive-shaft.
U.S. Pat. No. 3,266,090 (Gosney) discloses an improvement on the Snelling structure. It employs the same centrally positioned spring means, and also provides limited swivel means of the whole assembly with respect to the plane of the die-face.
U.S. Pat. No. 4,569,809 (Baggett) discloses an alignment jig mounted on a cutter assembly shaft, and its method of use to determine and adjust perpendicularity of a drive-shaft assembly controlling the cutter, with the die-face. Part of the jig includes a rod with a scraping edge to remove polymer drool, the rod being urged to the die-face with means such as a spring.
U.S. Pat. No. 4,614,307 (Lauser) also discloses a rotary melt cutter with axial adjustment of the cutting head.
U.S. Pat. No. 4,728,276 (Pauley et al.) discloses a rotary melt cutter which provides bearing means supporting the drive-shaft for axial adjustment. The adjustment means includes a support platform and support plate with a plurality of independently adjustable spring support members interconnecting them.
U.S. Pat. No. 4,874,307 (Comper et al.) discloses a rotary melt cutter where during operation, wear is eliminated because the cutting edges of the blades do not touch the die-face, but are spaced from it by a thickness at least equal to that of a hydrodynamic fluid film generated during operation.
U.S. Pat. No. 5,059,103 (Bruckmann et al.) discloses yet another type of spring-loaded hub structure interconnecting a drive-shaft and a cutter blade assembly.
U.S. Pat. No. 4,260,356 (Fujiwara et al.) discloses another melt cutter having a rotating shaft structure with at least one cutting tool with a blade, the shift being urged towards the extrusion block by a single elastic member such as a spring. The cutting tool is connected to the shaft using a universal joint which allows tilting of the cutting blades with respect to the extrusion block.
U.S. Pat. No. 4,021,176 (Dettmer et al.) discloses another design for a cutter apparatus with cutting knives resiliently tensioned towards the face of the die.
Each of these patents discloses different means of achieving conformity of knife-blades and die-face. Many incorporate various spring arrangements. However, none disclose the specific structural arrangements of the present invention.
In recent years, polymer compositions have become more complex and varied, and the nature of polymer melts is correspondingly more varied. Greater variations in viscosity and in other melt characteristics, resulting from new and varied additives, have become the norm. The Hale cutter, though adequate for the simpler compositions of the past, has been found inadequate to cope with many of these new compositions. Disruption of smooth continuous cutting often occurs. This can occur when lower cutter speeds are necesitated, because the hydraulic pressure is lower. Uneven polymer extrusion rate and surging can also cause disruption.
While changes in extruder die support and design, and use of new structural materials for extruders and dies can reduce all of the above problems, such an approach is expensive, involving major new equipment. There is a need for an improvement to rotary cutters, especially of the Hale design, which provide much greater control of knife-blade cutting-edge/dieface clearance, in order to provide good and uniform cutting of a wide variation of polymeric compositions.